Sepsis is the leading cause of death in critically ill patients, and treatment options are limited. Sepsis is often defined as a whole-body inflammatory state, leading to multiple organ failure. Our laboratory studies murine models of polymicrobial sepsis, and how immune dysfunction contributes to the disease's pathophysiology. The liver is often characterized as an immune tolerant organ, but is susceptible to sepsis because it induces an imbalanced inflammatory response. We have previously reported that CD8+ T cells in response to sepsis upregulate FasL (ligand) in the liver. Since activated leukocytes must travel through the endothelium to reach the liver, it is possible that activated lymphocyte populations mediate hepatocyte cell death, and ulitimately liver tissue injury. Liver sinusoidal endothelial cells (LSECs), an important cell population, form a thin barrier layer that separates leukocytes passing through the liver in the bloodstream from hepatoctyes. The primary function of LSECs may deem its behavior as a double-edged sword during sepsis: to protect hepatocytes from immune responses, while also to contain systemic inflammation. As a result of the interactions between activated T lymphocyte and LSEC populations, we hypothesize that activated T lymphocyte populations mediate apoptosis of neighboring endothelial cells in the liver, resulting in liver injury, inflammation, and damage. Our specific aims are to 1.) Determine the extent of how recruited CD8+ T cells induce changes to the LSECs, and 2.) Determine if innate regulatory, NKT and v5 T cells, impact LSEC's barrier function during sepsis. Our preliminary experiments have shown cells expressing CD31 (an endothelial cell marker) in the non-parenchymal cell population exhibit an increased level of Fas following cecal ligation and puncture (CLP), a surgical procedure to induce sepsis. Our future experiments include flow cytometry to further phenotype endothelial and T cell populations, purify T cells by using magnetic beads, quantify systemic cytokine levels via ELISA, perform apoptotic change analyses via western blotting and/or flow cytometry, and assess barrier function via electrical detection. The completed studies from this proposal will allow us to elucidate the immune pathological interactions at the molecular interface that occur during septic organ injury, and provide a novel therapeutic option for critically ill patients. Sepsis is the leading cause of death in critically ill patients. There are approximately 700,000 septic cases annually, and it has a mortality of 30%. We plan to use our proposed research strategies to discover a novel therapeutic target to understand and treat septic patients.